Method for producing a source of energy from an overpressured formation



EVAPORATORS x TIP.. .VA

I III TVI C. E. HOTTMAN Dn o T N E V N FIG. 2

DESCALER FIG. I

C. E. HOTTMAN Filed Feb. 7. 1965 METHOD FOR PRODUCING A SOURCE OF ENERGYFROM AN OVERPRESSURED FORMATION SEPARATOR Il III FLASH June 28, 1966United States Patent O "ice METHUD FOR PRODUCING A SOURCE F ENERGY FROMAN OVERPRESSURED FR- MATlQN Clarence E. Hottman, Houston, Tex., assignerto Shell @il Company, New York, NY., a corporation of Delaware FiledFeb. 7, 1963, Ser. No. 256,933 4 Claims. (Cl. 166-4) This inventionrelates to a method for utilizing energy provided by an overpressuredgeological formation. More particularly,` the invention pertains to amethod for locating an underground reservoir containing an aqueousliquid at a pressure and/ or temperature sufficient to yield usefulenergy, completing a well into the reservoir, and producing energyand/or chemicals obtained from the reservoir.

Energy provided by certain underground reservoir formations hasheretofore been utilized to produce electrical energy or to accomplishother useful work. However, such prior uses have been confined tooperations within regions in which the energy from the undergroundsources is available at or near a surface location. Such prior toutilizations have been confined to volcanic regions or other regionscontaining natural hot springs, geysers, or the like.

A principal object of the present process is to provide a method ofdiscovering and utilizing closed or isolated reservoir formations thatcontain aqueous liquids capable of releasing useful energy.

A further object is to provide a method of discovering over pressuredwater-bearing reservoirs in regions in which there is a shortage offresh water and utilizing such reservoirs to supply the Water,chemicals, and electrical energy, as a by-product.

A further object is to provide a method of utilizing energy provided byoverpressured geological formations by recompleting a well into anoverpressured waterbear ing reservoir that had been encountered andsealed off when the well was initially completed or abandoned.

A further object is to provide a method of discovering overpressuredwater-bearing reservoirs in regions in which it is desirable to producepetroleum and other materials from underground formations by means ofsteam and hot water drives or fluid-mining procedures, and utilizing theoverpressured reservoirs to supply heat and pressure energy tofacilitate such production operations.

This invention is directed to a method for completing a well into anunderground reservoir containing overpressured aqueous liquid that isentrapped by undercompacted shale and has a pressure and temperaturesufficient to yield useful energy. In this process, the earth formationsbeneath a selected area of land are investigated to determine thedistribution of undercom pacted shale formations andreservoirformations. The distribution of temperature is similarlydetermined if a certain temperature is desirable for the use to be madeof energy from the reservoir. Overpressured reservoir formations areidentified by comparing the above distributions and identifyingreservoir formations located below the top of undercompacted shaleformations at depths at which the temperature is sufcient for the use tobe made of the reservoir energy. An overpressured Patented .lune 28,i966 aqueous liquid bearing reservoir is identified by determiningthenature of the liquid contained in such an overpressured reservoir. Awell that encounters an overpressured aqueous liquid bearing reservoiris completed into that reservoir and equipped to produce fluids capableof releasing useful energy at a use location. Such a well is preferablyequipped to convey the aqueous liquid from the reservoir to a uselocation at substantially the temperature and pressure of the reservoirminus the pressure of the hydrostatic column between the reservoir andthe use location.

The aqueous liquid produced from an overpressured reservoir may be usedto perform various types of work, for example it may be used to generatelow pressure steam used to operate turbines or the like. In varioussituations the most economical use of such water would be as a feedstock for treating equipment such as water distillation plants. Suchdistillation plants can be designed to yield potable water andby-products such as energy and/or chemicals recovered from the watercontained in the reservoir. This invention is particularly directed to aAmethod for locating water useful as a feed stock for distillationplants in land areas where natural occurring fresh Water supplies arenot capable of supplying the demand. One such area is loacted in thelower Gulf Coast area of the United States. The land in this areaoverlies Undercompacted shale formations that entrap water Withinoverpressured water reservoirs of great size. The overpressured waterreservoirs located below the top of Undercompacted shale formations thatoccur in the Gulf Coast area frequently have a relatively low salinitythat is often less than sea water. Furthermore, since such waters can beproduced at the surface at temperatures in excess of 212 F. andpressures in excess of atmospheric pressure, the energy contained insuch water is suflicient to operate distillation plants without theaddition of other energy. Of course, it is appreciated that in certainsituations the use of some additional energy my be desirable to runvarious pumps and control devices associated with the distillation plantwhile the main source of energy for operating the plant is supplied bythe potential energy contained in the water supply.

The method of this invention and other advantages of the invention canbest be understood from the following detailed description of apreferred embodiment when taken in conjunction with the attacheddrawings wherein:

FIGURE l shows an arrangement for producing water suitable for feedstock for a water distillation system; and

FIGURE 2 shows an arrangement for producing water suitable for use in awater-steam secondary recovery operation.

Undercompacted shale formations occur in many geographical locations,particularly in the Texas-Louisiana Gulf Coast area of the United Statesthat have tertiary shale formations of great thickness. These shaleformations are usually deep water marine shales that contain few sandformations and are subject essentially to a uni-axial compaction as aresult of the compressive stress of the overburden. In order for thistype of shale formation to compact the fluids contained in the formationmust be removed. The fluids can only be removed by flowing into sandformations or other permeable avenues of escape.

Since the thick shale forma tions that occur in the Gulf Coast area havevery `few sand formations to act as avenues of escape the fluids areremoved at a much slower rate than from thinner shale formationssandwiched between sand formations. This inability of the uid to escapeyfrom the shale formation results in the creation of an abnormal iluidpressure within the formation.

The creation of abnormal pressures within a shale formation can be moreeasily understood by considering the following conception of a shalemodel. The shale model is formed from perforated metal plates which areseparated by springs and water with the complete structure beingenclosed within a cylindrical tube. The springs simulate thecommunication between the clay particles while the plates themselvessimulate the clay particles. Upon application of pressure to theuppermost plate the height of the springs between the plates remainsunchanged as long as no water escapes from the system. Thus, in theinitial stage the applied `pressure is supported entirely by the equaland opposite pressure of the water. As the water escapes from the systemthrough the perforations in the plate the uppermost plate will movedownward slightly and the springs will carry part of the applied load.As more water escapes the springs will carry an additional load untilfinally the complete axial load will be borne by the springs and thesystem will reach a state of equilibrium.

The clay particles forming the shale formation undergo a similarmovement to that described above for the model when subjected to auni-axial compaction due to the overburden. All formations are subjectto an axial compaction but more permeable formations reach equalibriummuch faster than shale formations. The inability of the shale formationsto reach equilibrium results in the occurrence of abnormally high uidpressured shale formations and abnormallyV high pressures in the fluidscontained in the permeable rocks that are enclosed in such shale. p

The existence of undercompacted shale formations beneath various areasof land has heretofore been known. When a well is drilled into areservoir formation located below the top of an undercompacted shaleformation, the reservoir fluid pressure is apt to exceed the pressureprovided by the drilling tluid unless the driller is employing arelatively high density mud, and such a well will kick or possibly blowout. Based on drillers reports, it was heretofore possible to determinethe areal extent of the undercompacted shale formations beneath selectedareas of land. However, such drilling data do not permit a determinationof the distribution of such shale formation since the reservoirformations on which these data are based may be located anywhere fromjust below the top to well below the undercompacted shale formations.

Methods have recently been developed for measuring properties of theearth formations beneath a selected area of land in a manner indicativeof the depth at which the tops of undercompacted shale formations areencountered below particular surface locations within the area. In onetype of such measuring procedures, wells disposed within the area arelogged to determine the rate of change with depth of a physical propertyof shale that is aifected by the density of the shale, anddeterminations are made of the depth at which the measured rateundergoes a change due to the encountering of an undercompacted shaleformation. Examples of logging procedures suitable for determining thedepths at which undercompacted shale formations are encountered includethe acoustic-logging procedures described in a copending application ofC. E. Hottman entitled Method for Determining Formation.Pressure, SerialNo. 144,685, tiled October 12, 1961, and the electrical loggingprocedures described in a copending application of C. L. Blackburn etal., entitled Method for Determining Formation Pressures, tiledSeptember 28, 1962, Serial No. 226,937.

In practicing the present invention, the localities and the depths atwhich undercompacted shale formations are encountered are measured todetermine the distribution of the undercompacted shale formations. Suchdeterminations of the distributions can be made by indicating thedistributions of the formations on maps of the underground formations,for example, by the contouring procedures that are conventionally usedin the contouring of the formations located by seismic, gravimetric, andthe like exploration techniques.

Techniques for investigating subsurface earth formations to determinethe distribution of reservoir formations and temperature are known tothose skilled in the art of petroleum exploration, and any means forobtaining such information can be used to obtain the determinations and/or indications utilized in the present process. Such techniques fordetermining the distribution of reservoir formations include seismic,gravimetric, electromagnetic, stratigraphic correlations, and the likeprocedures for investigating properties of subsurface formations. Inrespect to the distribution of the temperature of the subsurfaceformations, suitable techniques include conventional procedures forlogging wells dispersed within the area being investigated by means ofmaximum-reading,

continuous-recording, and the like temperature-measur-- ing equipment.

Overpressured reservoirs located below a selected area of land areidentied by comparing the distributions of the undercompacted shaleformations, reservoir formations, and temperature. Such identificationscan be made by simply overlaying plotted indications of thedistributions and indicating the reservoirs that lie below the top of anundercompacted shale formation at depths at which the temperature issuitable for the use to be made of uid in the reservoir. Alternatively,various digital and other data comparison techniques can be employed.

The properties of Overpressured reservoirs are further investigated toidentify such reservoirs that contain aqueous liquids. Suchinvestigations can be accomplished by-means of conventional welllogging, well-log correlating, and the like techniques, such as thoseinvolving comparisons of self potential, resistivity, conductivity,nuclear magnetism, and the like data that are affected by theelectrolyte concentration of the fluid in a reservoir. Suchinvestigations are preferably accomplished by measuring or samplingiluids contained in reservoirs that have been encountered by a well. Invarious instances, use can be made of wells that were previously drilledto encounter petroleum deposits and have penetrated into or through suchwater-bearing reservoir formations and were plugged back or cased off inthe portions encountering the water-bearing reservoirs.

The Overpressured water-bearing reservoirs that are particularlysuitable for use in the present process comp rise closed or isolatedreservoirs containing aqueous liquids entrapped by undercompacted shaleat pressures signicantly greater than the pressure of the hydrostaticcolumn above the reservoir and temperatures significantly greater than212 F. Such reservoirs are often relatively large, hot, and highpressured; eg., reservoirs having thicknesses as much as 500 feet overextents as much as to 500 square miles, temperatures as much as 365 F.or more, and pressures as much as 10,000 p.s.i.g. or more. The aqueousliquids in Overpressured watertbearing reservoirs frequently containdissolved or entrained materials that can be recovered as by-products ofthe present method of utilizing the energy contained in thosereservoirs. Such materials include dissolved and/or entrained organicmaterials, particularly petroleum materials, dissolved and/ or entrainedchemical elements such as sulfur, bromine, iodine, and the like;dissolved and/or entrained inorganic compounds; and the like.

After identifying an Overpressured water-bearing reseryoir that isencountered by a well, a well is completed into that reservoir and isequipped for conveying the reservoir water to a use location such as asurface location containing equipment designed for utilizing energyprovided by the pressure and temperature of the water. Normally thisconsists of casing the borehole drilled into the reservoir, and thendisposing suitable conduits within the casing for producing thereservoir aqueous liquid. The reservoir aqueous liquid is producedsubstantially at the formation temperature and at the formation pressureminus the hydrostatic head due to-the depth of the borehole.

Referring now to FIGURE 1 there is shown one embodiment of thisinvention in a simplified form for converting saline water to freshwater. The borehole 12 penetrates a shale formation and an overpressuredwater reservoir 11. The borehole 12 will normally be cased by means of acasing 13 that extends through the shale formation and the waterreservoir. The casing 13 is perforated at 14 by any of the variouscommercial means available. The water from reservoir 11 is then producedthrough the perforations and the production string 15 that is run intothe well.

The water at the surface is fed to a separator where the entrained gasis removed through a vent 21. The water then passes to a descaling unit22 where the entrained solids are precipitated and removed through aline 23. The 4recovered solids may be further processed to recover theminerals contained therein. The water then flows into the first stage 24of a multiple stage flash distillation type process. The pressure isreduced in the multiple stages and a certain percentage of the watervaporized in each stage. As is known the vapor from the rst stage 24 isconducted by a conduit 25 to the second stage 26 where it is condensedthus heating the feed supplied to the second stage by conduit 27. Thecondensed vapor from the first stage is essentially distilled water andis collected by a line 30. The remaining stages operate in the samemanner with the vapor from the preceding stage heating the feed for thenext stage. Of course for maximum efficiency a large number ofindividual stages as shown at 31 and 32 would be used as is well knownto those skilled in the art of constructing multiple effect flash typeplants.

Also shown in FIGURE 1 is a means for generating electricity usingeither the pressure and/or temperature of the water produced could beremoved through a con* duit 40 and the pressure of the water used tooperate a turbine 41. The quantitiy of water used to operate the turbine41 can be controlled by a valve 42 with valves 43 and 44 being used tocontrol the quantity of water supplied to the evaporator process andproduced by the well, respectively. After passing through the turbine 41the water passes into an evaporator 45 where a portion is flashed intosteam that is used to drive a second turbine d6. The exhaust from theturbine i6 is condensed and the condensate conveyed by a conduit 47 tojoin with the condensate from the evaporator process.

The following are examples of sources of energy found by the method ofthis invention:

Example 1 A selected area of land in Texas was investigated to determinethe distribution of undercompacted shale formations, reservoirformations, and temperature. The earth formations under that area werefound to include an underground shale formation, the top of which islocated at a depth of 9600 feet at a point at which the shale extendsabove a reservoir formation located at 13,000 feet in a zone at whichthe temperature is 365 F. the self potential and resistivity logs of awell encountering that reservoir indicated it to be a water-bearingreservoir. The well was completed into this overpressured water-bearingreservoir. This reservoir contains water entrapped by undercompactedshale and comprises a layer of sand having a thickness of about 200 feetand an extent from about 10 to 50 square miles, The water in thereservoir has a pressure of 10,400 p.s.i.g.

Equipping a well completed into this reservoir to transport reservoirfluids to a surface location for supplying superheated water as afeedstock to an evaporation apparatus of the type shown in FIGURE 1provides a means forconverting the reservoir water to fresh water. Thereservoir water which is so supplied has a pressure of 4,850 p.s.i.g., asalinity of 37,000 ppm. chloride ion concentration, an available flowrate of 1,300 barrels per day through an Sy-t-inch choke, and anestimated tubing flow rate of 10,000 barrels per day.

Example II A selected area of land under the Gulf of Mexico wasinvestigated to determine the distribution of undercompacted shaleformations, reservoir formations, and temperature. The earth formationsunder that area were found to include an underground shale formation,the top of which is located at a depth of 9,150 feet at a point at whichthe shale extends above a reservoir formation located at 11,000 feet ina zone at which the temperature is 285 F. The self potential andresistivity logs of a well encountering that reservoir indicated it tobe a waterbearing reservoir. The well was formation tested in thisoverpressured water-bearing reservoir interval. This interval containswater entrapped by undercompacted shale and comprises a net sandthickness of about 500 feet and an extent from about to 500 squaremiles. The water in this interval has a pressure of 8,250 p.s.i.g.

Equipping a well completed into this reservoir interval to transportreservoir fluids to a surface location for supplying superheated wateras a feedstock to an evaporation apparatus of the type shown in FIGURE 1provides a means for converting the reservoir water to fresh water. Thereservoir water which is supplied has a pressure of 2,750 p.s.i.g., anestimated salinity of 35,000 p.p.m. chloride ion concentration, and anestimated available flow rate of 100,000 barrels per day.

Referring to FIGURE 2 there' is shown an underground oil-bearingreservoir 50 conveniently located relative to the area of land mentionedin Example I. The reservoir contains about 50 percent of a pore volumeof residual oil at a pressure of about 2,000 p.s.i.g'. and may beproduced by utilizing energy provided by the overpressured water-bearingreservoir described in Example I. A well 51 is completed into theoverpressured water-bearing reservoir and equipped to transportsuperheated water from the reservoir into the injection tubing string 52into the oil-bearing reservoir. The pressure of the superheated water isreduced by means of a flow restriction 53 in the injection tubing stringso that some of the water evaporates. The flow restriction 53 may beprovided with surface operated controls in order that the quantity ofwater and steam injected may be varied. The oil displaced by theinjection of water and steam is produced from a production well 54completed into the oil-bearing reservoir.

Where such an oil-bearing reservoir has the permeability and/ or apressure such that it is unfeasible or undesirable to reduce thepressure of the hot water from an overpressured water-bearing reservoir,the energy provided by the overpressured reservoir can be usedadvantageously to supply all or part of the fluid-injecting andformationheating energy utilized in such a hot water drive. In general,the energy provided by such overpressured reservoirs can be utilized indisplacing fluid from underground earth formations by injecting a' fluidthat contains fluid from the overpressured reservoir into theunderground earth formation and transmitting energy from theoverpressured reservoir fluid to the fluid that is injected into theunderground earth formation.

From the above description it can be seen that the method of thisinvention comprises the steps of locating and completing a well into anoverpressured aqueous liquid bearing reservoir located below the top ofan undercompacted shale formation in a zone in which the temperature isrelativelyhigh and preferably above 212 F.; and producing the water fromthe reservoir to utilize energy from the reservoir. This method isfurther characterized as being particularly adaptable to providing freshwater obtained from underground reservoirs of salt water through the useof multiple effect processes in areas where sufficient natural suppliesof fresh water do not exist. This is particularly true since a knownlocation of undercompacted shale formations is the lower Louisiana-TexasGulf Coast area of the United States. This area also coincides with anarea that is finding it diicult to supply the fresh water demands of thearea. Thus, this invention would supply a low cost source of feed for amultiple effect process and reduce the net cost of the fresh waterproduced by the process so that it would compare favorable withpresently available commercial supplies of fresh water.

While the invention is particularly adapted to the production of freshwater in the lower Gulf Coast area of the United States itobviously'could be used to produce a source of energy for any desiredpurpose. The energy is derived from the elevated temperature andelevated pressure of the water produced by following the method of thisinvention. This energy can be used to generate electricity byconventional processes.

The method of this invention may be used to investigate previouslydrilled boreholes that were abandoned for failure to produce commercialquantitiesl of petroleum products. By using previously drilled boreholesthe net cost of the water produced'would be consideraly decreased. Thepreviously drilled boreholes can be investigated by the method of thisinvention and the wells can be recompleted into overpressured waterreservoirs. The water can then be produced and used as a sourceofpotential energy as described above.

I claim as my invention:

1. A method for locating and utilizing a source of potential energycomprising:

drilling at least one borehole in a selected region;

logging the borehole using a technique that responds to the density ofthe formations surrounding the borehole;

plotting with relation to depth the log data for the shale sectionsonly;

determining the depth at which the borehole has penetrated anundercompacted shale formation from the rate of change of the plottedlog data with depth; extending the borehole into at least one closedaquifer reservoir formation located below the determined depth in a zonewherein the minimum temperature exceeds a selected minimum temperature;and

completing at least one well into said reservoir and providing the wellwith conduits for conveying uid from the reservoir to a use location atsubstantially the pressure and temperature of the reservoir minus thepressure of the hydrostatic column between the reservoir and the uselocation.

2. The method of claim 1 wherein at least a portion of the producedfluid is converted to fresh water using the potential heat and pressureof the produced uid.

3. The method of claim 1 wherein the properties of an earth formationbelow a selective land area are measured to determine the location ofundercompacted shale formations and reservoir formations; v

comparing the distributions and identifying a reservoir formationlocatedl below. the top of the undercompacted shale formations; and

completing a well into the reservoir formation located below the top ofthe undercompacted shale formations.

4. The method of claim 1 wherein the potential pressure of the lluidproduced from said well is used to inject the produced Huid into asecond reservoir to displace the uid in the second reservoir.

References Cited by the Examiner UNITED STATES PATENTS 1,272,625 7/1918Cooper 166--68 2,230,001 l/1941 McConnell 166-9 2,258,615 10/1941Kendrick 166-68 2,736,381 2/1956 Allen 166-9 2,760,578 8/1956 Tausch166-45 2,973,811 3/1961 Rogers 166-9 X 3,134,438 5/1964 Huitt et al.166-45 3,177,940 4/1965 Ten Brink 166-45 X OTHER REFERENCES GeologicalAspects of Abnormal Reservoir Pressures in Gulf Coast Louisiana,Bulletin, American Association of Petroleum Geologists, vol. 37, No. 2(2/1953), pp. 410-432. (TN 860 A 5l.)

Methods of Exploring and Repairing Leaky Artesian Wells, Dept. of theInterior Publication, Water-Supply Paper 596-A, 4-1927.

Some Aspects of High Pressures in the D-7 Zone of the Ventura AvenueField, Transactions, AIME (1948), vol. 174, pp. 191-205. (TN 860 A 52.)

CHARLES E. OCONNELL, Primary Examiner.

C. H. GOLD, S. T. NOVOSAD, Assistant Examiners.

1. A METHOD FOR LOCATING AND UTILIZING A SOURCE OF POTENTIAL ENERGYCOMPRISING: DRILLING AT LEAST ONE BOREHOLE IN A SELECTED REGION; LOGGINGTHE BOREHOLE USING A TECHNIQUE THAT RESPONDS TO THE DENSITY OF THEFORMATIONS SURROUNDING THE BOREHOLE; PLOTTING WITH RELATION TO DEPTH THELOG DATA FOR THE SHALE SECTIONS ONLY; DETERMINING THE DEPTH AT WHICH THEBOREHOLE HAS PENETRATED AN UNDERCOMPACTED SHALE FORMATION FROM THE RATEOF CHANGE OF THE PLOTTED LOG DATA WITH DEPTH; EXTENDING THE BOREHOLEINTO AT LEAST ONE CLOSED AQUIFER RESERVOIR FORMATION LOCATED BELOW THEDETERMINED DEPTH IN A ZONE WHEREIN THE MINIMUM TEMPERATURE EXCEEDS ASELECTED MINIMUM TEMPERATURE; AND COMPLETING AT LEAST ONE WELL INTO SAIDRESERVOIR AND PROVIDING THE WELL WITH CONDUITS FOR CONVEYING FLUID FROMTHE RESERVOIR TO A USE LOCATED AT SUBSTANTIALLY THE PRESSURE ANDTEMPERATURE OF THE RESERVOIR MINUS THE PRESSURE OF THE HYDROSTATICCOLUMN BETWEEN THE RESERVOIR AND THE USE LOCATION.